A Redundant Arrays of Inexpensive Disks (RAID) apparatus is well known in the art as a storage apparatus equipped with a plurality of storage drives. A hard disk drive (HDD) is one example of a storage drive.
The HDD executes seeking of a target track using a magnetic head while a magnetic disk medium that is a storage medium is rotating at a certain rotating speed, and moves the magnetic head closer to the magnetic disk to magnetically record and play data.
Therefore, the processing speed is very slow compared to the processing speed of the CPU of the host computer since the writing and reading processing in the HDD is executed with mechanical operations. As a result, the HDD includes various functions to compensate for the discrepancy (performance limit) in processing speeds. A function called “write-back” is known as one of these functions.
A command issued by a host computer is received by a RAID apparatus controller that controls a plurality of HDDs and is stored in a queue in a job management table in a memory. The HDD is sequentially instructed to execute jobs waiting for processing registered in the job management table.
A controller of the RAID apparatus temporarily stores write data accompanying a write command in a cache memory when the command is a write command, and returns a completion report for the write command to the host computer at the time that the storing is completed (normal completion).
The write data temporarily stored in the cache memory is written in an HDD at a time when processing is available. Thus, the write-back function makes use of the feature that the storage operation of a cache memory is faster than that of an HDD. Also the performance limitations of the HDD may be alleviated.
However, write commands are issued by the host computer to the storage apparatus frequently and thus high load conditions on the storage apparatus may continue. In this case, write jobs waiting for processing to the HDDs accumulate in the job management table and there is a possibility that ultimately the storage apparatus may not be able to accept new commands and thus enter a queue full state.
Japanese Patent Laid-open No. 9-258907 and Japanese Patent Laid-open No. 2000-56932 are examples of related art.